1. FIRB Activities on lithium niobate: characterization of bulk materials and photoinduced effects Keypoints of our activities on LN “crystalline quality” - characterization methods Some examples Microstructures in LN by fs laser irradiation FIRB Project Microdevices in Lithium Niobate –Università di Pavia Electro-optic coefficients measurements Fe3+ EPR spectra microRaman

2. Characterization of structural, optical and electronic properties of LiNbO 3 crystals and substrates in connection with different growth processes and different doping Study of the transport phenomena and charge localization due to optical irradiation of LiNbO 3 (or other ABO 3 compounds, eventually doped) and of the irradiation effects on the linear and nonlinear optical properties Study of the feasibility of 1D, 2D and 3D periodical structures, waveguides and microstructures on LiNbO 3 (or other ferroelectric oxides) crystalline substrates by means of femtosecond laser irradiation in the transparent spectral region Keypoints FIRB Project Microdevices in Lithium Niobate –Università di Pavia Crystalline quality 123

3. How we study crystalline quality? Raman and micro-Raman spectroscopy Optical absorption, PL, TL, PC, TSC Hall, Photo-Hall and magneto-optical spectroscopy Ellipsometry Electron Paramagnetic Resonance (EPR) and Photo- EPR Static magnetization measurements Electro-optical characterization Femto-second laser sources *

4. Lattice of ideal, defect-free LN crystal coupling and mutual influence of intrinsic and extrinsic defects decrease of the intrinsic defect concentration FIRB Project Microdevices in Lithium Niobate –Università di Pavia Due to the Li-deficiency the conventional congruent crystals have high concentration of intrinsic (non-stoichiometric) defects, which can easily compensate a high concentration of extrinsic defects (for instance, optically or acoustically active impurities) Possibility to vary both the [Li]/[Nb] ratio and [O] contents (in addition to the modification by dopants!) is a very powerful tool for the optimisation of crystal parameters strong increase of the spectrum resolution due to line narrowing changes of some LN properties appearance of new impurity centers EPR Raman

5. EPR spectroscopy : Control of the material quality: check of purity of growth processes detection of defects and/or unwanted EPR active magnetic impurities information about structural disorder Evaluation of the oxidation state of the transition ions Information about site symmetry from the EPR signal angular dependence FIRB Project Microdevices in Lithium Niobate –Università di Pavia Fe3+ EPR lines (B//c) in CLN (LN:Fe 0.1%) …in quasi-st LN (LN:Fe 0.1%)

6. Raman in LiNbO 3 In crystals, Raman spectrum depends on the direction and polarization state of the incident and scattered light with respect to the cristallographic axes  Porto notation: k i (e i,e d )k d The crystal structure of pure LiNbO 3 has Rc3 space group symmetry and 4A 1 + 9E Raman-active modes are predicted by factor-group analysis FIRB Project Microdevices in Lithium Niobate –Università di Pavia

7. RS is strongly sensitive to orientation E light | c E light // c  -Raman to check disorientation, multidomains… FIRB Project Microdevices in Lithium Niobate –Università di Pavia

8. RS is sensitive to the deformation of the lattice and to the presence of point defects, becoming a powerful tool to deal with the problem of stoichiometry FIRB Project Microdevices in Lithium Niobate –Università di Pavia The mode at 880 cm-1 is the vibration, parallel to the c axis, of the oxygen ions which consists basically in the stretching of the Nb–O and Li–O bonds. When a Nb ion sits at a Li site its oxygen first neighbors increase their bonding forces respective to the perfect crystal situation because of the stronger electrostatic interaction.

9. RS can be used to check the stoichiometry (Li/Nb ratio) monitoring the changes of linewidth of some Raman modes. The fact that the linewidth of some Raman modes scale with the composition xc = [Li/([Li] + [Nb]) of LN crystals, together with the use of a confocal microscope (microRaman spectroscopy), allow a three dimensional estimation of the sample stoichiometry. FIRB Project Microdevices in Lithium Niobate –Università di Pavia

10.  Non-destructive stuctural tool  Micron-scale spatial resolution  Presence of a structurally disordered layer  Effectiveness of damage removal method  Control on optical surface finishing  Raman for surface quality analysis after wafering process:

11. Important complete characterization of: stoichiometry, nature and content of impurities, degree of structural disorder before starting with investigation of charge trapping mechanisms and phenomena related to photo-induced defects FIRB Project Microdevices in Lithium Niobate –Università di Pavia Study of the transport phenomena and charge localization due to optical irradiation of LiNbO 3 (or other ABO 3 compounds, eventually doped) and of the irradiation effects on the linear and nonlinear optical properties 2 Photovoltaic current, photoconductivity, Photo-EPR vs %, doping,  T

12. Study of the feasibility of 1D, 2D and 3D periodical structures, waveguides and microstructures on LiNbO 3 (or other ferroelectric oxides) crystalline substrates by means of femtosecond laser irradiation in the transparent spectral region 3 “MICROSTRUCTURAL MODIFICATION OF LINBO 3 CRYSTALS INDUCED BY FEMTOSECOND LASER IRRADIATION” Appl. Surf. Science in press FIRB Project Microdevices in Lithium Niobate –Università di Pavia

13. Activity of Pavia Unit in fs-laser writing Ti:Sapphire oscillator (25 nJ-  130 fs-82 MHz) Laser system 2 (low energy, high repetition rate): Amplified Ti:Sapphire (1 mJ-  130 fs-1 kHz) Laser system 1 (high energy, low repetition rate): femtosecond irradiation of congruent LN as a function of pulse energy, exposure time, exposure depth, crystal orientation, etc.. characterisation via in situ optical microscopy and a posteriori micro- Raman spectroscopy energy deposition through multi-photon absorption  energy transfer strongly depends on pulse intensity FIRB Project Microdevices in Lithium Niobate –Università di Pavia

14. Microstructures in LiNbO 3 crystals by fs laser irradiation (laser 1) b)50-  m-diameter hole in a z-cut CLN plate (laser 1, 10 s, 50  J, 63x microscope objective lens) c)same as a) imaged by a polarizing microscope a)2-  m-diameter holes in a z-cut CLN plate (laser 1, 10 ms, ~1  J, LWD 50X microscope objective lens) a) b) c) FIRB Project Microdevices in Lithium Niobate –Università di Pavia

15. Microstructures in LiNbO 3 crystals by fs laser irradiation (laser 2) a)125-  m-diameter hole in a z-cut CLN plate (laser 2, 30s, 10 nJ, 63x microscope objective lens) b)same as a) imaged by a polarizing microscope. A bright zone aside the hole is visible due to photo-induced birefringence. FIRB Project Microdevices in Lithium Niobate –Università di Pavia

16. Micro-Raman investigation of microstructures formed by laser 2 635 cm -1 880 cm -1 z(xx)z Raman spectra recorded at positions 1 to 4 as shown in the top left side image. A1 symmetry - forbidden Raman lines appear as approaching the edges of the microstructure indicating some orientation changes in the crystal structure 1 2 3 4 20  m 2D mapping of Raman intensity in the square The brighter the colours the larger the Raman line intensity Image of a hole in z-cut CLN plate (laser 2, 0.01s, 5 nJ, 20x objective lens). The maximum depth of the hole is 10  m FIRB Project Microdevices in Lithium Niobate –Università di Pavia 1 2 3 4

17. Micro-Raman investigation of microstructures formed by laser 1 1 2 3 10  m Raman spectra recorded in zone 1 to 3. The main E- type peaks are strongly quenched while the A1 peak at 635 cm -1 increases. In spectrum 3 even Nb-O related vibrations at frequency larger than 500 cm -1 are absent, as it would happen in an amorphous layer Image of a microstructure in a z-cut CLN plate (laser 1, 10s, 300  J, 63x microscope objective lens) FIRB Project Microdevices in Lithium Niobate –Università di Pavia

18. Conclusion FIRB Project Microdevices in Lithium Niobate –Università di Pavia femtosecond irradiation induces disorder in the crystal structure causing the appearance of Raman peaks of forbidden symmetry niobium oxides are formed in the ablation process with laser system 2 amorphous surfaces are present in the region ablated by means of laser system 1 High-intensity ultra-short pulses from laser system 1 probably leads to the formation of an electron plasma and localized optical breakdown, whereas charge accumulation and photorefractive-like damage may be the mechanism excited in the case of the high-repetition-rate-low-energy fs pulses from laser system 2. Ablation edges of microstructures formed by laser system 2 are smooth and a strong induced birefringece is present all around. In both cases multi-photon absorption is the path for energy transfer into the medium

19. EO coefficients of Lithium Niobate CLN (x c =0.485) SLN (x c =0.500) rTcrTc 17.5 1 20.5 7,10 (20  1) 14,15 19.9 18,20 (18  1) 15 r T 13 (11  1.0) 2,3,8,9 (10.49  0.07) 6 (6.28  0.07) 11 (9.25  0.07) 16 (10.4  0.8) 19 10.5 9 r T 33 (34.0  2.5) 2 (31.5  1.4) 8,3,4,9 (31.4  0.2) 6 (29.4  0.2) 16 (38.3  1.4) 19 37 9 Class 3m r 33, r 13 = r 23, r 22 = -r 12 =-r 61, r 42 =r 51, r c =r 33 -(n o /n e ) 3 r 13 r in pm/V at =633nm Measuring techniques may rely on ellipsometry or interferometry with DC or AC applied electric field Constant stress (r T ) or constant strain (r S ) EO coefficients are measured when the electric field frequency is below/above the acoustic resonance of the crystal (above 500KHz) FIRB Project Microdevices in Lithium Niobate –Università di Pavia i =1,6j =1,3

20. EO coefficients of Lithium Niobate EO coefficients of Crystal Technology CLN (empty symbols) and SLN (full symbols) with x c =0.497 provided by the Crystal Growth laboratory- Universidad Autonoma de Madrid FIRB Project Microdevices in Lithium Niobate –Università di Pavia r 33 rcrc r 13

21. EO coefficients of commercial SLN SLN wafer from OXIDE Co. Japan with nominal x c =0.50 (kindly provided by project partner AVANEX) FIRB Project Microdevices in Lithium Niobate –Università di Pavia rTcrTc r T 13 r T 33 r T c = r T 33 -(n o /n e ) 3 r T 13 19.49.630.719.8 r in pm/V ± 5%, =633nm from AC field ellipsometry from AC field interferometry OXIDE data sheet: r 33 =38.3 r 13 = 10.4

22. Contradictory results from the literature K. Chah et alii, APB 67 (1998) 65 Y. Kondo et alii, J.JAP 39 (2000) 1477 FIRB Project Microdevices in Lithium Niobate –Università di Pavia Different trends were measured for LN EO coefficient as a function of 100. x c

23. Femto-second laser writing and sculpturing The extremely high power density (> TW/cm 2 ) of focussed fs pulses easily excites multi-photon absorption avalanche ionization optical breakdown h IR EvEv EcEc leading to ablation or refractive index changes in transparent media A lot of work in glass but still few examples in LN Long penetration depth and low thermal damage open the way to microstructuring in bulk materials Perspective of micro-channels and holes, 3D gratings, buried waveguides, 3D directional couplers etc FIRB Project Microdevices in Lithium Niobate –Università di Pavia